We studied an ensemble of negatively charged boron vacancies created in high-purity hexagonal boron nitride single crystals, by implanting 2.5 keV helium ions with a dose of \( 2 {\rm E13/cm^2} \) . These centers can be addressed via our home-built confocal photoluminescence microscope using 532 nm laser excitation. ODMR spectra of \( V_B^- \)  defects were realized by applying microwaves. Tape-exfoliated hBN flakes with a thickness of tens of nanometers were directly transferred to a gold-film coplanar microwave waveguide surface. The coplanar microwave waveguide was fabricated by photolithography on an insulating substrate surface to achieve spin control and plasmonic-enhanced high ODMR contrast (Fig.REF a) [1]}. Here the microwave drive field can make Rabi frequency up to 40 MHz. ODMR spectra were performed by sweeping the frequency under continuous optical illumination and measuring \( I_{\rm {PL}} \) . When the microwave driving field is not resonant with a spin transition, the laser polarizes the \( V_B^- \)  defects into the \( m_S=0 \)  spin state, resulting in a maximum value of \( I_{\rm {PL}} \) . In contrast, when microwave driving field is resonant with a transition between the \( m_S = 0 \)  and \( m_S = \pm 1 \)  levels, the \( I_{\rm {PL}} \)  is reduced due to the lower photoluminescence rate in \( m_S = \pm 1 \)  [2]}. If the optical transition is spin conserving, spin rotations in both the ground state and the excited state can be detected [3]}. Fig.REF b shows a typical continuous wave (cw) ODMR measurement at B = 0 G. In addition to the well-known two resonant peaks, corresponding to the transitions between the ground state \( m_S = 0 \)  and \( m_S = \pm 1 \) , which are detected near 3.48 GHz, there is also a relatively broad resonant peak near 2.1 GHz, and the broad peak splits to two peaks when external magnetic field is applied. We tentatively attribute them to spin transitions in the orbital excited state.
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